Recovery of nitrogen and oxygen



F. KOEHLER Filed Ad 24, 1938 2 Sheets-Sheet 1 I .l lllllllll-lIIILII-III'Ir-IIIIIII'II 4 Franz Kathi-r "Hi6 ATTORNEYS June 12?,194'? macovmmr OF mmoemn AND oxmml Patented June 17, 1947 aacovnnx orNITROGEN AND OXYGEN Franz Koehler, Leuna, Germany; vestedin the AttorneyGeneral, of the United States Application August 24, 1938, Serial No.226,591

. In Germany August 28, 1937 Sections 3am! 14, Public Law 690, August a,1946. Patent expires August 28, 1957 2 Claims.

The present invention relates to improvements in and apparatus for therecovery of nitrogen and oygen.

It is already known that by liquefaction of air at high pressure andwhile using counter-current tubular heat-exchangers and by rectificationof the, liquefied mixture, practically pure nitrogen and oxygen can beprepared. This method has two considerable drawbacks, however, namely-(1) that a great part of the air to be separated into its constituentshas to be brought to high pressure and (2) that the whole of the airmust be carefully purified from water and carbon dioxide before theliquefaction in order to avoid stoppages in the counter-current tubularheatexchangers.

By reason of these drawbacks, another process has been adopted for someyears in which considerably lower pressures are used and in which it isunnecessary to purify the whole of the air from water vapor and carbondioxide before the liquefaction. This method differs mainly-from thatalready mentioned in that the heat-exchange is not efiected by leadingthe gas to be cooled.

and the gas absorbing heat in counter-current to each other in tubularexchangers but in socalled recuperators which are cooled periodically,

whereby the low heat content necessary to cover the loss of low heatcontent is produced by releasing from pressure a small amount (fromabout 4 to 10 per cent of the whole) of air compressed to high pressure(as for example 200- atmospheres) and if necessary by releasing frompressure one of the gases to be recovered in expansion engines. Byreason of the advantages attending this method, it has acquired a greatimportance for the recovery of oxygen, although by the said method it isimpossible to obtain pure nitrogen as well as the pure oxygen.

I have now found that by the last-mentioned method both nitrogen andoxygen of high purity can be prepared by withdrawing from the centralpart of the upper of the two columns used for.

rectification a small amount of a mixture of nitrogen and oxygen andusin its low heat content within the system.

In other processes for separating air into its would be deemed injuriousbecause considerable amounts of nitrogen and oxygen are necessary forthe removal of the substances (carbon dioxide and water) separatedin therecuperator from the gases which are not purified according to the saidprocess.

In spite of this, however, pure nitrogen and oxygen are obtainedaccording to this invention by the withdrawal of a certain amount of amixture of nitrogen and oxygen at a central part of the upper column,when the amount of highly compressed, preliminarily purified air isincreased as a substitute for the withdrawn gas mixture and as anadditional source for the cooling. This additional highly compressed airprovides the additional amount of nitrogen and oxygen necessary for thecontinuous removal of the impurities from the recuperator. Moreover, byreleasing the pressure on the same, so much low heat content is producedthat the whole of the nitrogen of the lower column can be used as atrickling liquid for the upper column. In this way there can berecovered in the same apparatus not only oxygen of a high degree ofpurity but also nitrogen of a high degree of purity.

The production of the additional highly compressed and preliminarilypurified air, according to this modification, requires extra energy andtherefore involves extra expense. This may be avoided according toanother modification of theinvention by combining the measures usualpart of the nitrogen of the lower column in exconstituents in which arecuperator is not used,

it has already been proposed to withdraw a mixtureof nitrogen and oxygenat'a central part of thefractionation column in order to obtain purernitrogen and p'urer oxygen. It could not have been expected, however,that this measure would be of use in' a method using a recuperator inwhich any removal of gas mixture from the system pansion machines, onlya relatively small amount of a mixture of nitrogen and oxygen need bewithdrawn from the central part of the upper column and additional largeamounts of preliminarily purified, highly compressed'air are notnecessary.

There isthus the advantage that-only a simple apparatus, a smallexpenditure of energy and the use of low pressures for the major portionofthe air are necessary and that a preliminary purification of the majorportion of the air is unneces- The mixture of oxygen and nitrogen can bewithdrawn from the rectification column ata point at which it containsfrom about 5 to 35, preferably from 15 to 25 per cent of oxygen. Bywithdrawing such a mixture there is produced in the rectification columnsuch equilibrium relation that the amount of trickling liquid (nitrogen)present is sufficient for washing the mixture of nitrogen and oxygeneven when a part of the nitrogen has been withdrawn from the lowercolumn for the cooling in pressure-release machines.

The mixture of nitrogen and oxygen withdrawn from the upperrectification column serves in particular to precool the highlycompressed air used for the additional refrigeration. This highlycompressed air is released to the pressure prevailing in therectificaton column and introduced into the upper or lower rectificationcolumn for utilization of the low heat content thus produced.

The highly compressed air, after the precooling by the withdrawn mixtureof nitrogen and oxygen, may be further cooled by heat exchange with thatpart of the air or of the nitrogen which is withdrawn from therectification column in the manner known in the low pressure process inorder to be released from pressure in an expansion engine to about thetemperature of liquid air.

A specially advantageous modification of the present invention consistsin adding the mixture of nitrogen and oxygen withdrawn from the middleof the upper rectificationcolumn, after subjection to heat exchange, tothe air to be separated into its constituents. Since this added mixtureof nitrogen and oxygen is free from water and carbon dioxide, thecontent of water and carbon dioxide in the air to be separated into itsconstituents is reduced. It is then readily possible for thesesubstances, which are deposited by the cooling of the air in therecuperators, to be further removed in the next working operation by thepure oxygen and the pure nitrogen without it being necessary to increasethe amount of 'highly compressed air purified from water and carbondioxide supplied for the further production of low heat content to themixture to be separated into its constituents in the rectificationcolumn, in order to render certain the sublimation of the depositedsubstances in the recuperators.

In some cases it may be advantageous to compress the gas mixturewithdrawn from the central part of the upper rectification column tohigh pressure after the heat exchange and to use it instead of thehighly compressed air serving for the cooling by releasing it frompressure if desired in expansion engines. In this way the amount of airwhich has to be freed from water and carbon dioxide before its coolingis reduced.

Another way in which the low heat content of the gas mixture withdrawnfrom the upper rectification column may be utilized consists in usingthe mixture for the cooling of one or more additional recuperators whichare arranged in parallel with the pair of recuperators through which thenitrogen and oxygen flow and also serve for the precooling of the airunder low pressure to be separated into its constituents, whereby thevalves "may be actuated by the existing regulation device of the pair ofrecuperators.

Three embodiments of apparatus in accordance with this invention willnow be described with reference to the accompanying drawings, but

the invention is not restricted to the particular embodiments shown.

A, B, C and D are recuperators which are operated alternately in knownmanner, E is a twocolumn apparatus for separting air into itsconstituents, F is a counter-current heat exchanger, J is a low pressureair turbo-compressor, R is a pressure release valve and K is a highpressure air compressor.

The air compressed by the compressor J to about 4.5 atmospheres iscooled in the recuperators A, B, C and D almost to, the liquefactiontemperature and introduced through a pipe a into the lower column of theapparatus E for rectification. Furthermore a certain amount of aircompressed to about 200 atmospheres and freed from Water and carbondioxide is led through a pipe b and the heat exchanger F and releasedfrom pressure through the pressurerelease valve R into the lowerrectification column. The cold constituents (oxygen and nitrogen) areled through pipes c (nitrogen) and 11 (oxygen) into the coldaccumulators A, B, C,

D from which they escape through c and d after absorbing heat. Fromabout the middle of the upper rectification column, there is withdrawnthrough e a small amount of a mixture of nitrogen and oxygen which isled to the counter-current heat exchanger F to the highly compressed airintroduced at b.

The embodiment according to Figure 2 differs from that according toFigure 1 in that for the production of additional low heat contentnitrogen under a pressure of about 4.5 atmospheres is withdrawn from thelower rectification column and. supplied through a pipe I to acounter-current exchanger G in which it absorbs part of the heat of thehighly compressed air precooled in F, This nitrogen is then releasedfrom pressure in the expansion engine H and led into the coldaccumulators A and B together with the nitrogen escaping from the upperrectification column. In Fig. 3 the mixture of oxygen and nitrogenwithdrawn through line e from the middle part of the upper rectificationcolumn in this embodiment is compressed by compressor K, after flowingthrough heat exchanger F, and used instead of the highly compressed airserving for the cooling by releasing it from pressure with the aid ofpressure release valve R.

What I claim is:

1. Process of rectifying air into its major constituents which comprisesrefrigeratingair without liquefaction at a slightly elevated pressure,passing the refrigerated air into a rectifying column, introducing arelatively small quantity of highly compressed air into said columnwhile releasing the pressure thereon to thereby efi'ect rectification ofthe air into its major constituents, separately withdrawing saidconstituents, passing said constituents in heat exchange relationship tothe air undergoing refrigeration, withdrawing a relatively small amountof a mixture of nitrogen and oxygen from said rectifier, utilizing itslow heat content and recycling the same.

2. Apparatus for rectifying air into its major constituents whichcomprises a plurality of refrigeration chambers, a two-column rectifyingvessel, means for forcing air through said refrigeration chambers andinto said rectification vessel, means for forcing a comparatively sma .1quantity of highly compressed air into the rectification vessel, meansfor reducing the pressure of said highly compressed air upon itsintroduction into the rectification vessel. means for sepa- 5 6 ratelyremovingsubstantially pure nitrogen and REFERENCES CITED oxygen from therectification vessel, means for conducting the same in heat eirchangerelation- The .followmg ref rences are of record in th ship through therefrigeration chambers, means file of this Patent! for withdrawing amixture of nitrogen and oxy en 5 UNITED STATES PATENTS from therectification vessel and means for re- 'cycling said mixture. NumberName Date a FRANZ KOEHLER v 2,009,08 Gomonet July 23, 1935 2,048,076Linde July 21, 1936

